diff --git a/Makefile b/Makefile
index e1bf8414..baf7493f 100644
--- a/Makefile
+++ b/Makefile
@@ -135,6 +135,7 @@ include watch-faces.mk
 
 SRCS += \
   ./movement.c \
+  ./movement_activity.c \
 
 # Finally, leave this line at the bottom of the file.
 include $(GOSSAMER_PATH)/rules.mk
diff --git a/movement.c b/movement.c
index 8fe408b3..cf978a4e 100644
--- a/movement.c
+++ b/movement.c
@@ -43,6 +43,8 @@
 #include "lis2dw.h"
 #include "tc.h"
 #include "evsys.h"
+#include "delay.h"
+#include "movement_activity.h"
 
 #include "movement_config.h"
 
@@ -170,6 +172,10 @@ static void _movement_handle_top_of_minute(void) {
             movement_request_sleep();
         }
     }
+
+    if ((date_time.unit.minute % 5) == 0) {
+        _movement_log_data();
+    }
 #endif
 
     // update the DST offset cache every 30 minutes, since someplace in the world could change.
diff --git a/movement_activity.c b/movement_activity.c
new file mode 100644
index 00000000..ea2d9d6f
--- /dev/null
+++ b/movement_activity.c
@@ -0,0 +1,76 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2025 Joey Castillo
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#include "movement.h"
+#include "movement_activity.h"
+#include "tc.h"
+#include "thermistor_driver.h"
+
+#ifdef HAS_ACCELEROMETER
+
+// RAM to stash the data points.
+movement_activity_data_point movement_activity_log[MOVEMENT_NUM_DATA_POINTS] = {0};
+// the absolute number of data points logged
+uint32_t data_points = 0;
+
+// hacky: we're just tapping into Movement's global state for stationary detection.
+// do we need better API for this? i'm less bothered now that it's all in Movement.
+extern uint8_t stationary_minutes;
+
+void _movement_log_data(void) {
+    size_t pos = data_points % MOVEMENT_NUM_DATA_POINTS;
+    movement_activity_data_point data_point = {0};
+
+    // Movement tracks stationary minutes when deciding whether to sleep.
+    data_point.bit.stationary_minutes = stationary_minutes;
+
+    // orientation changes are counted in TC2. stash them in the data point...
+    data_point.bit.orientation_changes = tc_count16_get_count(2);
+    // ...and then reset the number of orientation changes.
+    tc_count16_set_count(2, 0);
+
+    // log the temperature
+    thermistor_driver_enable();
+    float temperature_c = thermistor_driver_get_temperature();
+    thermistor_driver_disable();
+    // offset the temperature by 30, so -30°C is 0, and 72.3°C is 102.3
+    temperature_c = temperature_c + 30;
+    if (temperature_c < 0) temperature_c = 0;
+    if (temperature_c > 102.3) temperature_c = 102.3;
+    // now we have can fit temperature into a 10-bit value
+    data_point.bit.measured_temperature = temperature_c * 10;
+
+    /// TODO: log light level
+
+    // log the data point
+    movement_activity_log[pos].reg = data_point.reg;
+    data_points++;
+}
+
+movement_activity_data_point *movement_get_data_log(uint32_t *count) {
+    *count = data_points;
+    return movement_activity_log;
+}
+
+#endif
diff --git a/movement_activity.h b/movement_activity.h
new file mode 100644
index 00000000..68f0f2ee
--- /dev/null
+++ b/movement_activity.h
@@ -0,0 +1,52 @@
+/*
+ * MIT License
+ *
+ * Copyright (c) 2025 Joey Castillo
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in all
+ * copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
+ * SOFTWARE.
+ */
+
+#pragma once
+
+#ifdef HAS_ACCELEROMETER
+
+#include <stdint.h>
+
+// Log 36 hours of data points. Each data point captures 5 minutes.
+#define MOVEMENT_NUM_DATA_POINTS (36 * (60 / 5))
+
+typedef union {
+    struct {
+        uint32_t stationary_minutes: 3;
+        uint32_t orientation_changes: 9;
+        uint32_t measured_temperature: 10;
+        uint32_t measured_light: 10;
+    } bit;
+    uint32_t reg;
+} movement_activity_data_point;
+
+/// @brief Internal function, called every 5 minutes to log data.
+void _movement_log_data(void);
+
+/// @brief Returns a pointer to the data log.
+/// @param count is a pointer to a uint32_t. The absolute number of data points logged is returned by reference.
+/// You can assume that log[count % MOVEMENT_NUM_DATA_POINTS] is the latest data point, and work backwards from there.
+movement_activity_data_point *movement_get_data_log(uint32_t *count);
+
+#endif
\ No newline at end of file
diff --git a/watch-faces/sensor/activity_logging_face.c b/watch-faces/sensor/activity_logging_face.c
index a6d58010..07da1eec 100644
--- a/watch-faces/sensor/activity_logging_face.c
+++ b/watch-faces/sensor/activity_logging_face.c
@@ -27,56 +27,24 @@
 #include "activity_logging_face.h"
 #include "filesystem.h"
 #include "watch.h"
-#include "tc.h"
-#include "thermistor_driver.h"
+#include "movement_activity.h"
+#include "watch_utility.h"
 
 #ifdef HAS_ACCELEROMETER
 
-// hacky: we're just tapping into Movement's global state.
-// we should make better API for this.
-extern uint8_t stationary_minutes;
-
-static void _activity_logging_face_log_data(activity_logging_state_t *state) {
-    watch_date_time_t date_time = movement_get_local_date_time();
-    size_t pos = state->data_points % ACTIVITY_LOGGING_NUM_DATA_POINTS;
-    activity_logging_data_point_t data_point = {0};    
-
-    data_point.bit.day = date_time.unit.day;
-    data_point.bit.month = date_time.unit.month;
-    data_point.bit.hour = date_time.unit.hour;
-    data_point.bit.minute = date_time.unit.minute;
-    data_point.bit.stationary_minutes = stationary_minutes;
-    data_point.bit.orientation_changes = tc_count16_get_count(2); // orientation changes are counted in TC2
-
-    // write data point. WARNING: Crashes the system if out of space, freezing the time at the moment of the crash.
-    // In this exploratory phase, I'm treating this as a "feature" that tells me to dump the data and begin again.
-    if (filesystem_append_file("activity.dat", (char *)&data_point, sizeof(activity_logging_data_point_t))) {
-        printf("Data point written\n");
-    } else {
-        printf("Failed to write data point\n");
-    }
-
-    // test for off-wrist temperature stuff
-    thermistor_driver_enable();
-    float temperature_c = thermistor_driver_get_temperature();
-    thermistor_driver_disable();
-    uint16_t temperature = temperature_c * 1000;
-    if (filesystem_append_file("activity.dat", (char *)&temperature, sizeof(uint16_t))) {
-        printf("Temperature written: %d\n", temperature);
-    } else {
-        printf("Failed to write temperature\n");
-    }
-
-    state->data[pos].reg = data_point.reg;
-    state->data_points++;
-
-    // reset the number of orientation changes.
-    tc_count16_set_count(2, 0);
-}
-
 static void _activity_logging_face_update_display(activity_logging_state_t *state, bool clock_mode_24h) {
-    int8_t pos = (state->data_points - 1 - state->display_index) % ACTIVITY_LOGGING_NUM_DATA_POINTS;
     char buf[8];
+    uint32_t count = 0;
+    movement_activity_data_point *data_points = movement_get_data_log(&count);
+    int8_t pos = (count - 1 - state->display_index) % ACTIVITY_LOGGING_NUM_DATA_POINTS;
+    watch_date_time_t timestamp = movement_get_local_date_time();
+
+    // round to previous 5 minute increment
+    timestamp.unit.minute = timestamp.unit.minute - (timestamp.unit.minute % 5);
+    // advance backward by 5 minutes for each increment of state->display_index
+    uint32_t unix_timestamp = watch_utility_date_time_to_unix_time(timestamp, movement_get_current_timezone_offset());
+    unix_timestamp -= 300 * state->display_index;
+    timestamp = watch_utility_date_time_from_unix_time(unix_timestamp, movement_get_current_timezone_offset());
 
     watch_clear_indicator(WATCH_INDICATOR_24H);
     watch_clear_indicator(WATCH_INDICATOR_PM);
@@ -94,21 +62,21 @@ static void _activity_logging_face_update_display(activity_logging_state_t *stat
         if (clock_mode_24h) {
             watch_set_indicator(WATCH_INDICATOR_24H);
         } else {
-            if (state->data[pos].bit.hour > 11) watch_set_indicator(WATCH_INDICATOR_PM);
-            state->data[pos].bit.hour %= 12;
-            if (state->data[pos].bit.hour == 0) state->data[pos].bit.hour = 12;
+            if (timestamp.unit.hour > 11) watch_set_indicator(WATCH_INDICATOR_PM);
+            timestamp.unit.hour %= 12;
+            if (timestamp.unit.hour == 0) timestamp.unit.hour = 12;
         }
         watch_display_text(WATCH_POSITION_TOP_LEFT, "AT");
-        sprintf(buf, "%2d", state->data[pos].bit.day);
+        sprintf(buf, "%2d", timestamp.unit.day);
         watch_display_text(WATCH_POSITION_TOP_RIGHT, buf);
-        sprintf(buf, "%2d%02d%02d", state->data[pos].bit.hour, state->data[pos].bit.minute, 0);
+        sprintf(buf, "%2d%02d%02d", timestamp.unit.hour, timestamp.unit.minute, 0);
         watch_display_text(WATCH_POSITION_BOTTOM, buf);
     } else {
         // we are displaying the number of accelerometer wakeups and orientation changes
         watch_display_text(WATCH_POSITION_TOP, "AC");
         sprintf(buf, "%2d", state->display_index);
         watch_display_text(WATCH_POSITION_TOP_RIGHT, buf);
-        sprintf(buf, "%-3u/%2d", state->data[pos].bit.orientation_changes > 999 ? 999 : state->data[pos].bit.orientation_changes, state->data[pos].bit.stationary_minutes);
+        sprintf(buf, "%-3u/%2d", data_points[pos].bit.orientation_changes > 999 ? 999 : data_points[pos].bit.orientation_changes, data_points[pos].bit.stationary_minutes);
         watch_display_text(WATCH_POSITION_BOTTOM, buf);
     }
 }
@@ -118,10 +86,6 @@ void activity_logging_face_setup(uint8_t watch_face_index, void ** context_ptr)
     if (*context_ptr == NULL) {
         *context_ptr = malloc(sizeof(activity_logging_state_t));
         memset(*context_ptr, 0, sizeof(activity_logging_state_t));
-        // create file if it doesn't exist
-        if (!filesystem_file_exists("activity.dat")) {
-            filesystem_write_file("activity.dat", "", 0);
-        }
     }
 }
 
@@ -157,9 +121,6 @@ bool activity_logging_face_loop(movement_event_t event, void *context) {
                 _activity_logging_face_update_display(state, movement_clock_mode_24h());
             }
             break;
-        case EVENT_BACKGROUND_TASK:
-            _activity_logging_face_log_data(state);
-            break;
         default:
             movement_default_loop_handler(event);
             break;
@@ -172,14 +133,4 @@ void activity_logging_face_resign(void *context) {
     (void) context;
 }
 
-movement_watch_face_advisory_t activity_logging_face_advise(void *context) {
-    (void) context;
-    movement_watch_face_advisory_t retval = { 0 };
-
-    // log data every 5 minutes
-    retval.wants_background_task = (movement_get_local_date_time().unit.minute % 5) == 0;
-
-    return retval;
-}
-
 #endif
diff --git a/watch-faces/sensor/activity_logging_face.h b/watch-faces/sensor/activity_logging_face.h
index 489b4d53..90bf0093 100644
--- a/watch-faces/sensor/activity_logging_face.h
+++ b/watch-faces/sensor/activity_logging_face.h
@@ -41,23 +41,9 @@
 
 #define ACTIVITY_LOGGING_NUM_DATA_POINTS (36)
 
-typedef union {
-    struct {
-        uint32_t day: 5;
-        uint32_t month: 4;
-        uint32_t hour: 5;
-        uint32_t minute: 6;
-        uint32_t stationary_minutes: 3;
-        uint32_t orientation_changes: 9;
-    } bit;
-    uint32_t reg;
-} activity_logging_data_point_t;
-
 typedef struct {
     uint8_t display_index;  // the index we are displaying on screen
     uint8_t ts_ticks;       // when the user taps the LIGHT button, we show the timestamp for a few ticks.
-    int32_t data_points;    // the absolute number of data points logged
-    activity_logging_data_point_t data[ACTIVITY_LOGGING_NUM_DATA_POINTS];
 } activity_logging_state_t;
 
 void activity_logging_face_setup(uint8_t watch_face_index, void ** context_ptr);
@@ -71,7 +57,7 @@ movement_watch_face_advisory_t activity_logging_face_advise(void *context);
     activity_logging_face_activate, \
     activity_logging_face_loop, \
     activity_logging_face_resign, \
-    activity_logging_face_advise, \
+    NULL, \
 })
 
 #endif // HAS_TEMPERATURE_SENSOR